A liquid crystal display panel used in a television receiver (referred to in the following as a liquid crystal TV display) includes a matrix array of horizontally oriented scanning electrodes and vertically oriented signal electrodes, the signal electrodes and scanning electrodes being disposed on mutually opposing substrates of the liquid crystal display panel. Selection signals are successively applied to the scanning electrodes, while stepwise modulated signals are applied to the signal electrodes, to display a television picture. Each scanning electrode is selected during a single selection interval, with a relatively lange amplitude voltage being applied to the selected electrode. At the same time, signals at a relatively small amplitude voltage level, stepwise modulated in accordance with the video data contents, are applied to all of the signal electrodes. As a result, picture elements disposed between the selected scanning electrode and the opposing signal electrodes are driven into the display state. Since with a liquid crystal display panel it is necessary to apply the modulated drive signals simultaneously to all of the signal electrodes during each selection interval, the composite video TV signal must be sampled prior to selection of each scanning electrode and the resultant sample signals then subjected to A/D conversion. In this way, stepwise-varying modulation data required during at least one selection interval is generated and stored immediately prior to that interval. Generally speaking, each selection interval usually corresponds to one horizontal scanning interval (designated in the following as 1H). However, depending upon the scanning method employed it is possible for each selection interval to correspond to 2H. Sampling operations are usually performed during each selection interval to generate modulation data which will be required during the succeeding selection interval. Display of a complete TV picture is accomplished by successive selection of all of the scanning electrodes in this way.
When an NTSC format image is displayed by such a liquid crystal TV display, each frame consists of two fields, where one field consists of 262.5 scanning lines. However due to the time interval required for the vertically flyback and:for the correct display region the effective number of scanning lines is approximately 220. In the case of a liquid crystal TV display, interlace of the first and second fields is generally not performed, and identical selection operations are carried out for each field in a frame. Furthermore, due to the characteristics of liquid crystal material obtainable at the present time, it is only possible to obtain an acceptable value of contrast ratio if the matrix drive time-sharing factor is limited such that the number of scanning lines is held to slightly more than 100. Various methods have been proposed to lower the effective value of this time-sharing factor, to provide a display having 220 scanning lines in each field. One such method uses duplex matrix driving, whereby each scanning electrode has a width equal to two picture elements, with two signal electrode drive systems being employed (i.e. making the number of signal electrodes equal to twice the number of rows of picture elements). In this way, the stepwise video modulation signals can be applied to two scanning electrodes simultaneously during each selection interval. This enables a display having 220 scanning lines to be provided, by using only 110 scanning electrodes. Another method of lowering the effective time-sharing factor is the upper/lower split-display method, in which the display panel is divided into two parts, i.e an upper and a lower display section. In this way the scanning electrodes and signal electrodes are respectively divided into two sets of electrodes in the upper and lower display sections. Thus, driving a display of this type which has 220 scanning lines is equivalent to driving a conventional display having 110 scanning lines.
The duplex method described above has the disadvantage that the electrode patterns are complex, resulting in a lowering of the manufacturing yield for such a display panel, and hence increased cost. In addition, due to the fact that connecting leads must be positioned between pairs of adjacent picture elements, the ratio of the area of the picture elements to the total display area image quality. In the case of the upper/lower split-display method on the other hand it is difficult to suitably configure the electrodes at the boundary region between the upper and lower display sections, causing a lowering of manufacturing yield. Furthermore, the number of integrated circuits necessary for display drive operation is increased, with this method, resulting in increased cost of manufacture. In addition, differences between the images produced by the upper and lower display sections can occur, leading to a lowering of image quality. Because of the various problems described above, the only practical method of driving a liquid crystal TV display in use at present is to simply reduce the number of rows of picture elements by half, i.e. to employ only 110 rows of picture elements, by comparison with the capability of the duplex drive method or the split-display method described above which can enable 220 rows of picture elements to be driven.
In the following, a horizontally aligned row of picture elements will be referred to as a horizontal picture element line, for precision of description. With the duplex matrix drive method described above, two scanning lines are produced over each horizontal picture element line line, as compared with non-duplex operation in which each horizontal picture element line line corresponds to one scanning line. With the simple 110-row drive method, every seconc scanning line of the total 220 lines is subjected to A-D conversion to produce stepwise-modulated video data, i.e. a total of 110 lines are converted. It is also possible to perform A-D conversion of all of the 220 scanning lines, with such a simple non duplex display drive method, but in either case only 110 scanning lines are actually displayed. As a result, with such a display technique not only is there a loss of display resolution, by comparison with the basic capability of the TV signal (440 scanning lines per frame), but in addition the resolution is further reduced by one-half due to the fact that interlace scanning is not utilized, in addition to the lowered resolution resulting from the use of only one line of each successive pair of scanning lines during each field (i.e. each pair of scanning lines being displayed by one row of picture elements). Thus the 440 scanning lines per frame of the TV signal are displayed such that four lines are averaged into one scanning line, with a total of only 110 scanning lines per frame being displayed.
FIG. 1 is a diagram showing a prior art arrangement of drive electrodes for a liquid crystal TV display having n.times.m picture elements, in which T.sub.11 to T.sub.n1 denote n scanning electrodes, and S.sub.1 to S.sub.m are m signal electrodes. As described above, n is approximately 110. FIG. 2 is a timing chart showing the waveforms of the composite video signal and the selection signals applied to the scanning electrodes, for the 110-row liquid crystal TV display of FIG. 1. In FIG. 2, VIDEO denotes the television composite video signal, with portions of two successive fields being shown. CL denotes a signal which determines the timings of selection of the scanning electrodes. A signal produced by pulse-width modulation of signal CL in accordance with the video data is applied to drive the signal electrodes.
TP1, TP2 and TP3 denote selection signals which are applied to the scanning electrodes T.sub.11, T.sub.21, T.sub.31, shown in FIG. 1, and are of positive polarity at certain timings and negative polarity at other timings, i.e. the polarity which is midway between the maximum positive and negative potentials of these signals is the non-selection potential and selection of a scanning electrode occurs when one of signals TP1, TP2, . . . is set to the positive or negative potential value thereof. As shown in FIG. 2, the selection signals applied to mutually adjacent scanning electrodes are of opposite polarity, and the polarity of the selection potential applied to each scanning electrode alternates in successive fields, to apply AC drive to the liquid crystal display panel.
As described above, with prior art scanning methods it has only been possible to produce an NTSC standard television image using a liquid crystal TV display, having either of two basic numbers of scanning electrodes, i.e. approximately 220 or 110 electrodes. This is due to the fact that the number of output terminals available from an integrated circuit chip employed for a scanning electrode drive circuit is only approximately 110, while in addition the characteristics of presently available liquid crystal material are such that satisfactory display contrast can only be obtained in practice with a drive time-sharing factor of slightly more than 100. However problems arise with regard to poor display quality if the number of scanning electrodes is limited to approximately 110, and this loss of quality is especially noticeable when characters of diagrams are displayed, since the displayed image appears extremely rough. In the case of a liquid crystal display panel having 220 scanning electrodes, the duplex drive method can be employed to hold the drive time-sharing factor to that of a 110-line display, without reduction of display contrast. Alternatively, two integrated circuits can be employed to drive two sets of drive electrodes, by the split-display method described above. However in either case problems including increased manufacturing cost will arise.
It is an objective of the present invention to overcome the problems described above, to provide an improved method of driving a liquid crystal TV display, which provides both high display quality and low manufacturing cost.